Landscape Structure and Species Interactions Drive the Distribution of Salmon Carcasses in Coastal Watersheds

Abstract

The disproportionate effects of some species can drive ecosystem processes and shape communities. This study investigates how distributions of spawning Pacific salmon within streams, salmon consumers, and the surrounding landscape mediate the distribution of salmon carcasses to riparian forests and estuaries. This work demonstrates how carcass transfer can vary spatially, within and among watersheds, through differences in pink (Oncorhynchus gorbuscha) and chum (O. keta) salmon distributions within 16 streams on the central coast of British Columbia over a five-year period. Spawning pink salmon concentrated in the lower reaches of all streams, whereas chum salmon shifted from lower to upper stream reaches as the area of spawning habitat increased. Salmon carcasses transferred to riparian areas by gray wolves (Canis lupus) were concentrated in estuaries and lower stream reaches, particularly shallow reaches of larger streams surrounded by large meadow expanses. Black and grizzly bears (Ursus americanus and U. arctos) transferred higher numbers and proportions of salmon carcasses to riparian areas compared to wolves, transferred more carcasses in areas of higher spawning density, and tended to focus more on chum salmon. Riparian subsides were increasingly driven by bear-chum salmon associations in upper stream reaches. In addition, lower proportions of salmon carcasses were exported into estuaries when densities of spawning salmon were lower and spawning reaches of streams were longer. This study demonstrates how salmon subsidies vary between and within watersheds as a result of species associations and landscape traits, and provides a nuanced species-specific and spatially explicit understanding of salmon-subsidy dynamics.

Spawning salmon density influences fruit production of salmonberry (Rubus spectabilis)

“…Specifically, we expected that higher salmon densities would correlate with more fruits, which would also be larger, sweeter, and have more seeds. We also predicted that chum salmon ( Oncorhynchus keta ) would have a greater effect than pink salmon ( O. gorbuscha ), since they are transferred to riparian areas at higher rates in our study area (Hocking and Reimchen 2006, Harding et al 2019).…”

“…Chum salmon are also transferred to terrestrial ecosystems at a higher rate than pink salmon (Reimchen 2000, Hocking and Reimchen 2006). Bears are responsible for approximately 75% of the salmon transfer from water to land in coastal British Columbia (Reimchen 2000) and they prefer chum over pink salmon (Andersson and Reynolds 2018, Harding et al 2019). This is thought to be because chum salmon are larger than pink salmon (3.1 and 1.05 kg, respectively), making it easier for bears to access energy‐rich parts and providing a greater reward (Andersson and Reynolds 2018).…”

“…At each stream, we observed salmonberry traits at two levels: the number of fruits per shrub, and the characteristics of these fruits—weight, seed count, and estimated sugar content (° Brix). Higher spawning salmon densities result in increased carcass deposition in riparian areas (Harding et al 2019), where carcasses can have a fertilizing effect. As such, we predicted a positive correlation between salmon density and each of the shrub and fruit‐level traits that we evaluated.…”

Fear the reaper: ungulate carcasses may generate an ephemeral landscape of fear for rodents

“…But, carrion in natural systems involve complexity that has not been explored in great detail. For example, species interactions can have a feedback on how carcass biomass is distributed on the landscape [ 80 ] and be responsible for the structure of ecosystems [ 81 ]. Understanding carcass provisioning, resulting landscapes of fear, and their effect on ecosystems and populations is lacking.…”

Stable Isotopes Reveal Variation in Consumption of Pacific Salmon by Brown Bears, Despite Ready Access in Small Streams

“…provides an important source of nutrients for a wide variety of organisms (Willson and Halupka 1995;Gende et al 2002). Notwithstanding this diversity of consumers, brown bears Ursus arctos and black bears U. americanus are uniquely capable of catching and killing a large fraction of the salmon in small streams (Quinn et al 2017), and thereby influencing the distribution of marine nutrients in nearby terrestrial and aquatic ecosystems (e.g., Hilderbrand et al 1999a;Meehan et al 2005;Reimchen 2017; reviewed by Helfield and Naiman 2006;Harding et al 2019). Salmon are generally considered a paramount prey source for bears because these anadromous fish are a rich source of energy, contributing to growth, reproductive success, and population density of bears (Farley and Robbins 1995;Hilderbrand et al 1999bHilderbrand et al , 1999cFortin et al 2007;Van Daele et al 2013).…”